Diameter Chevrolet 216 pistons?

[Lynn Eades]

Hi Alex. I'm not sure how much you know, so don't be offended if you know all this.
If I recall correctly, ).010" wear, or 0.003" oval, and it's time for a rebore. Those old tollerances were the deal back in the day, for a patch up,but today they're hardly acceptable.

Piston to bore clearance is such that the bore is honed so that the piston slips on a 0.002" feeler, but locks up on a 0.003" feeler. (feeler sitting down the thrust face of the bore)
So a 0.002" piston to bore clearance.
The piston is measured on the thrust face (90 degrees to the gudgeon pin) just below the oil ring. This is the biggest measurement you get on a cam ground piston.
My info covers 1935 to 1951, but does not say anything about the difference between cast iron and alluminium pistons. Each should have a different spec.
Measuring the bottom of the bore will tell you what the bore size was, but what you need to know is what the maximum measurement is. That tells you what you have to do. For example if you have 0.012" wear, you need to find some 0.020"O.S. pistons, for your motor re-conditioner to bore and finish hone to.
For me, I would buy some good quality modern piston rings for it.